The present invention relates to a process for the manufacture of high molecular weight poly-(ethylene terephthalate) in two stages.
It is known to manufacture high molecular weight poly-(ethylene terephthalate) by reacting dialkyl esters of terephthalic acid with diols. The industrial manufacture is generally carried out in two stages.
In the first stage, the dialkyl ester of terephthalic acid is first converted, using trans-esterification catalysts, to the bis-(2-hydroxyalkyl) terephthalate or to low molecular weight precondensates thereof. This is done by reacting a dialkyl ester of terephthalic acid with ethylene glycol in the presence of 35 to 290 ppm of manganese, in the form of a manganese-II salt, together with 6 to 95 ppm of cobalt, in the form of a cobalt-II salt, as the catalyst, at temperatures in the range of 170.degree.-220.degree. C. to give a low molecular weight precondensate and then adding 45 to 140 ppm of phosphorus in the form of a compound.
The precondensate thus formed is then subjected, in the second stage, to a condensation reaction in the presence of polycondensation catalysts, to give high molecular weight poly-(ethylene terephthalate), high temperatures and low pressure being employed for this stage. That is, the complete condensation of the precondensate is effected in the presence of catalysts at temperatures in the range of 270.degree.-290.degree. C. and under low pressure, to give high molecular weight poly-(ethylene terephthalate).
In order to obtain a reaction time which is practicable industrially, both the trans-esterification reaction and the polycondensation reaction must be accelerated by catalysts. However, the commonly used catalysts promote not only the reaction by which the polyester is built up but also degradation reactions during trans-esterification and polycondensation. As a result, they effectively determine essential characteristics of the polyester, such as color, melting point and stability to heat. Little is known about the influence of the type and amount of the catalysts on the stability of the polyesters to hydrolysis.
Usually, different catalysts are employed for each of the two reaction stages. Typical catalysts include, for example, salts of the metals lead, cadmium, calcium, magnesium, zinc, cobalt and manganese.
Zinc salt catalysts have the disadvantage that they strongly catalyze degradation reactions of the polyester during the trans-esterification and polycondensation and thus impair the characteristics of the end product (Faserforschung und Textiltechnik 24, 445 (1973)).
Manganese, lead, cadmium and cobalt compounds discolor the finished poly-(ethylene terephthalate) (Faserforschung und Textiltechnik 13, 481 (1962)). On the other hand, calcium and magnesium compounds, although they give colorless products, have a lower catalytic reaction (Polymer 16, 185 (1975)). These adverse effects of the trans-esterification catalysts can be avoided to some extent during the polycondensation in a known manner by the prior addition of phosphorus compounds (Faserforschung und Textiltechnik 19, 372 (1968); and British Patent Specifications Nos. 588,833 and 769,220).
The polycondensation catalysts conventionally employed are antimony, titanium or germanium compounds.
Antimony compounds have the disadvantage that a partial reduction to metallic antimony takes place in the course of the polycondensation, so that the condensation products have a grayish tinge (H. Ludewig, Polyesterfasern (Polyester Fibres), Akademie-Verlag, Berlin 1975; and German Offenlegungschrift 2,126,218).
The use of titanium compounds in the conventional concentrations results in yellow-colored products in the case of poly-(ethylene terephthalate) (J. Polymer Sci. (A), 4, 1851 (1966)).
Germanium compounds as polycondensation catalysts, do indeed produce colorless polycondensation products. However, unfortunately they accelerate the polycondensation reaction to a lesser extent than, for example, antimony compounds. Longer reaction times are therefore required, and as a result of this, the characteristics of, for example, the resultant poly-(ethylene terephthalates) are impaired (Faserforschung and Textiltechnik 13, 481 (1962) and German Offenlegungsschrift 2,107,247).
In addition to the use of these catalysts, as single components, combinations of different catalysts have also already been used, especially in order to improve the color of the polyester (German Offenlegungsschriften 2,126,218 and 2,548,630).
However, the use of, for example, poly-(ethylene terephthalate) for the manufacture of monofilaments for filter fabrics has shown, in experiments carried out in-house, that the products produced with the known catalysts or catalyst combinations are very severely degraded when they come into contact with a hot aqueous filtrate, as a result of which the filter fabric loses strength and becomes mechanically unusable.
German Offenlegungsschrift 2,707,832 discloses a catalyst mixture of manganese, cobalt, phosphorus, titanium and antimony compounds. With the exception of the phosphorus compound, all the catalyst components are added to the reaction mixture at the start of the first stage (trans-esterification). The poly-(ethylene terephthalates) obtained by this process are strongly colored and possess low stability to hydrolysis and heat.
A catalyst combination of manganese and cobalt compounds is known for the first stage (trans-esterification) from British Patent Specification No. 1,135,233.
German Offenlegungsschrift 2,020,330 describes the addition of carbodiimides to polyesters which have already completely condensed, before these polyesters are further processed. It is said that, inter alia, improved stability to hydrolysis is achieved by this means. However, it has been found that these polyesters are also discolored.